1. Field of the Invention
The present invention relates to a slow wave structure for a traveling wave tube which provides low variation in signal gain versus frequency and expanded bandwidth.
2. Description of the Related Art
In a traveling wave tube (TWT), a stream of electrons is caused to interact with a propagating electromagnetic signal or wave in a manner which amplifies the electromagnetic wave. In order to achieve such interaction, the electromagnetic wave is propagated through a slow wave structure, such as a conductive helix wound around the path of the electron stream, or a folded waveguide type of structure in which a waveguide is effectively wound back and forth across the path of the electron stream. For effective interaction, the slow wave structure is designed to propagate the electromagnetic wave with an axial phase velocity approximately equal to the velocity of the electron stream.
The main components of a conventional TWT are illustrated in FIG. 1. The TWT is generally designated as 10, and includes an electron gun 12 which generates and feeds the electron stream into a slow wave structure 14. The electron stream is guided through the slow wave structure by means of a static magnetic focusing field and is captured at the other end of the slow wave structure 14 by an electron collector unit 16. The electromagnetic wave is fed into the slow wave structure 14 through a radio frequency input coupler 18, and led out of the structure 14 through a radio frequency output coupler 20.
The slow wave structure 14 provides a path for propagation of the electromagnetic wave which is considerably longer than the axial length of the structure 14, whereby the electromagnetic wave is made to propagate through the slow wave structure 14 at a phase velocity which is approximately equal to the propagation velocity of the electron stream. The interactions between the electrons in the stream and the traveling wave cause velocity modulation and bunching of electrons in the stream. The net result is a transfer of energy from the electron stream to the electromagnetic wave traveling through the slow wave structure 14, and exponential amplification of the traveling wave.
TWTs are highly useful for amplification of signals at microwave, and more recently, millimeter wave frequencies, for communications, radar, and numerous other applications. The present invention especially relates to a TWT which employs a folded waveguide type slow wave structure including a plurality of coupled cavities, such as disclosed in U.S. Pat. No. 3,010,047, entitled "TRAVELING-WAVE TUBE", issued Nov. 21, 1961, to D. Bates.
The electron stream slows down in velocity as it gives up energy to the traveling wave. As a result, the traveling wave and the electron stream progressively lose synchronization, with the electron stream lagging behind the traveling wave. Eventually, the electron bunches are no longer favorably phased to give up energy to the traveling wave, and the amplification process ceases. Further amplification may be obtained by providing the slow wave structure 14 with a "velocity taper" section which progressively slows down the traveling wave to match the reduction in axial velocity of the electron stream.
FIG. 2 illustrates the slow wave structure 14 as being of the coupled cavity type, including a driver stage 22 and an output section 24. The driver stage 22 is subdivided into an input section 26 and a center section 28 by a sever section 30. The sever section 30 is provided to prevent the generation of reflected waves which could result in oscillation, and typically includes a high loss material which absorbs substantially all of the traveling wave while enabling the velocity modulated electron stream to pass therethrough unaffected. The electron stream entering the center section 30 generates a new traveling wave, which itself interacts with the electron stream to produce more signal gain.
Another sever section 32 which provides the same function as the sever section 30 is disposed between the driver stage 22 and the output section 24. The output section 24 typically includes a primary section 34, which operates at substantially the same phase velocity as the driver stage 22, to overcome losses introduced by the severs 30 and 32 and provide a strong input signal for a velocity taper section 36. The section 36 is designed to operate at a reduced phase velocity and may include several subsections (not shown) to match the phase velocity reduction of the traveling wave to the axial velocity reduction of the electron stream.
The sections 26, 28, 34 and 36 have essentially similar configurations. FIG. 3 illustrates a representative portion of any one of these sections which includes a plurality of hollow spacers 38 alternating with discs 40. The discs 40 separated by the hollow spacers 38 define cavities 42 therebetween, and have arcuate slots 44 formed therethrough for coupling adjacent cavities 42 together. The discs 40 further have a central hole 45 for passage of the electron stream and may be formed with central drift tubes 46 on either side. The drift tubes 46 enhance the interaction between the electromagnetic wave and the electron stream.
With reference also being to FIG. 4, the discs 40 are assembled in an alternating manner such that the slots 44 of adjacent discs 40 are inverted by 180.degree. relative to each other. The resulting configuration constitutes a folded waveguide, having an effective length greater than the axial length of the structure 14. The phase velocity in the slow wave structure 14 may be reduced by reducing the spacing between adjacent discs 40, and vice-versa. Although not shown, the structure 14 is further provided with suitable means for confining the electron stream within the central axial hole 45, such as a periodicpermanent-magnet (PPM) arrangement as disclosed in the above referenced patent to Bates.
A traveling wave tube of conventional design has a small signal gain characteristic curve which decreases parabolically from a maximum value at a particular frequency. The signal gain variation is generally quite large, and is especially undesirable in millimeter-wave TWTs where the performance band is a small fraction of the total cold passband due to weak interaction between the traveling wave and electron stream. The cold passband is the frequency range between the lower and upper cavity mode cutoff frequencies of the TWT. The large signal gain variation and associated narrow performance band cause high bit error rates in TWTs used in communication systems as described in an article entitled "Bit-Error-Rate Testing of High-Power 30-GHz Traveling-Wave Tubes for Ground-Terminal Applications", by K, Shalkhauser, in IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-34, No. 12, December 1987, pp. 2625-2633.
Although it is theoretically possible to flatten the signal gain variation using gain equalizers, these are expensive, time consuming to use, not readily available at millimeter-wave frequencies, and often introduce phase distortion.